The agricultural and forestry wastes in China are more than 880 million tons at present. Due to the lack of effective utilization technology, the agricultural and forestry wastes have to be incinerated or abandoned. Such a practice not only wastes resources, but also causes a large number of socio-economic and ecological problems such as haze weather, air pollution, soil mineralization, fire disasters and traffic accidents; it has become the focus and difficult problem attracting public concern. The existing approaches of comprehensively utilizing biomass are quite extensive, and many approaches have high resource utilization rate and economic benefits, such as papermaking, direct-fired power generation, fiber ethanol and biogas; however, because of the small utilization scale and consumption amount, low efficiency, pollution or specific geographical requirements, any one approach cannot fundamentally meet the requirements of clean utilization of biomass with high quality and added-value. The rapid pyrolysis refining technology has emerged in the past three decades, the technology is used for converting biomass into liquid which has large energy density and can be easily stored and transported, the liquid may be processed to produce alternative fuels for vehicles, liquid fuels and chemical raw materials, it is the most promising and environmentally-friendly substitute for petroleum. The technology does not subject to restrictions of production scale and consumption territory, it can meet the requirements of large scale, high efficiency, high added-value, and being clean and pollution-free, it has been generally recognized as “the biomass energy technology with the greatest industrial development potential in the century”. It is not only the focus and hotspot of biomass energy research & development in the international community, but also the frontier exploration project in the new energy field of the strategically emerging industry in China.
The internationally representative processes are mainly consisting of the rotary cone reaction process of Twente University in the Netherlands, the carrying bed reaction process of the Georgia Institute of Technology in the United States of America (USA), the bubbling and circulating fluidized bed process of the Ensyn Engineer Association of the Canada, the vortex reaction process developed by the National Renewable Energy Laboratory (NREL) of the USA, the multi-layer vacuum pyrolysis grind reaction process of the Laval University in the Canada. Similar technologies have been developed in sequence by a variety of domestic institutions in China. However, due to the defects and deficiencies existed in the processing technology, only the rotary cone process and the bubbling and circulating fluidized bed process have achieved 10,000-ton level industrial production at present. In the above processes, only the circulating fluidized bed rapid pyrolysis process can utilize the heat generated by partial semi-coke circulating combustion generated by pyrolysis to meet the heat demand of the reaction, it has the advantages such as the energy utilization is reasonable, the liquid yield is high, the equipment is simple and it can be easily upsized and expanded to a large scale, it is the top research project of the biomass energy field at home and abroad, and it is considered as the biomass rapid pyrolysis process which is most likely to achieve industrialized production. However, the circulating fluidized bed pyrolysis generally requires fluidizing gas, the cooling load is heavy, and it is difficult to remove fine powder generated by fluidized wear from the oil. The downflow circulating fluidized bed has the advantages such as it is provided with the parallel flow and descending movement along the gravity field, the solid-solid or gas-solid contact time is short, the reaction is fast with uniform radial distribution and small back-mixing, and the solid/gas or solid/solid ratio may be flexibly adjusted. It maintains the advantages of high liquid yield and large-scale production of fluidized bed pyrolysis, and it does not need for fluidized wind, the residence time of pyrolysis gas is short, the particle wear and fine powder is less, thereby lowering the cooling load and energy consumption, and the removal of ash from oil is relatively easy, thus the downflow circulating fluidized bed will be the optimum reactor for rapid pyrolysis of biomass which shall be urgently developed and applied. However, given that the biomass raw material contains potassium and a high water content, and there is a significant density difference between the biomass raw material and the pyrolysis semi-coke and carrier, the current typical rapid pyrolysis process in the world generally confront with eleven difficult problems affecting the industrial upsizing and stable operation in long period of the biomass pyrolysis technology, namely caloric transfer and reaction control, the oil carried with ash, high water content in oil, low liquid yield, oil and gas coking and blockage, semi-coke carrier density return, self-polymerization in the process of heating biological oil, the dilution of dry gas by fluidized gas and energy consumption, the melting of potassium-containing carrier causes the bed material to coke and deactivate the bed, the drying of biomass raw materials, abrasion of mechanical moving parts of reactors under high temperature, safe discharge of semi-coke and clean utilization.
CN101481619A discloses a self-mixing downflow fluidized bed rapid pyrolysis process for solid organics, wherein the mixing descending reactor and the fluidized bed re-activator are coupled to form a reaction-regeneration coupling system of the solid heat carrier circulation, which performs control on the caloric transfer and reaction. Despite the difficult problem of oil carried with ash caused by the hot ash is eliminated from the source by fractionation of heat carrier, pyrolyzing heat carrier with the large and medium particles with the downflow pyrolysis reactor, the discharge of hot ash with fine particles; however, the biomass powder is not fractionated after the drying process, the fine particles therein will further cause the phenomenon of oil carried with ash from the source. The oil and gas are not directly fractionated, and the problem of high water content in oil has not been effectively solved. In addition, the industrialized promotion of the self-mixing downflow fluidized bed rapid pyrolysis process for biomass has been plagued by the difficult problems such as oil and gas coking and blocking, semi-coke carrier density return, self-polymerization in the process of heating biological oil, semi-coke volatiles unregulated, semi-coke powder efflux easy to catch fire, semi-coke not be cleaned and utilized, and drying of the biomass raw materials.